The present invention is directed to a nozzle for use in a rotational casting machine used for applying one or more coats of liquid elastomer, such as polyurethane, to a rotating body, such as a pipe, cylinder, and the like, whereby an elastomer covering or coating is applied to the exterior or interior of the pipe, cylinder, or the like. The body being coated may be used in steel or paper mills, or many other industries, in order to protect the body proper during end-use, as well as for providing other desired properties. Rotational casting machines, that rotationally mount a body to be coated with polyurethane or other liquid elastomer, are disclosed, for example, in U.S. Pat. Nos. 5, 601,881 and 5,658,386—Grimm, et al., and include a translational and vertically-adjustable mixing head in which is formed the polyurethane to be used for coating the body. Polyurethane chemicals such as polyols, isocyanates, catalysts, etc. are metered to the mixing head. In this process the liquid materials are dispensed onto the body being coated and react very quickly to produce the solid polyurethane that will cover or coat the body. The hardness of the elastomer-coating is controlled by the types of polyols used and their mixture-ratio, along with the corresponding adjustment of the amount of isocynate added to the mixture in the mixing head, in order to obtain hardness in both Shore A to Shore D ranges. The hardness desired for the elastomer depends upon many factors, such as end-use of the body being coated.
A considerable problem with rotational casting machines is the trade-off of forming a liquid polyurethane having a desired viscosity and reactivity in order to prevent run-off or dripping of the applied elastomer from the body being coated during the coating process, and the need to prevent the clogging of the dispensing head attached to, and forming part of, the mixing head during the coating-application process. If the viscosity is made too great or reactivity too fast, then the dispensing head tends to become clogged faster, requiring more frequent down-time in order to unclog and clean the dispensing head. Presently-used dispensing heads, such as that disclosed in above-mentioned U.S. Pat. Nos. 5,601,881 and 5,658,386, are sheet-die extruders or nozzles, which sheet-die nozzles are provided with an exit slot the width of the nozzle, in order to ensure that a wider swath of coat-application is applied. However, the problem with these prior-art dispensers is that each hypothetical section of the liquid elastomer exiting the dispensing head at the exit thereof has not, typically, had the same dwell-time in the dispensing nozzle along the width and the length thereof, whereby there is not ensued that the exothermically formed elastomer has the same properties throughout when applied to the body to be coated. Minimum dwell-time and uniform discharge from the nozzle in order to ensure equality and sameness of properties throughout is a highly desirable property in order to prevent build up, hardening or curing of the liquid elastomer therein and the concomitant clogging of the nozzle and exterior build up of whiskers or “stalactites” due to differential residence-time of the material in the nozzle. Moreover, the height and width of the slit of these sheet-die nozzles are dependent upon the viscosity and/or the reactivity of the material being dispensed, thus necessitating the replacement of one sheet-die with another one having a different slit-height and slit-width when materials of differing viscosity/reactivity are used. However, even changing sheet-dies in order to accommodate materials of different viscosity/reactivity in order to prevent frequent clogging of the sheet-die in order to obtain the desired coating thickness, has still not solved the problem of the frequent clogging and associated frequent down-times when sheet-die nozzles are used. This may be attributed to the fact that the flow of the material in the dispensing nozzle is not laminar, causing variation in dwell-time of the liquid in the nozzle, such that the dwell-time for some segments of the liquid are greater than a required minimum, leading to at least partial solidification of those segments in the interior of the nozzle. Over time, a build-up of solidified material develops, causing clogging at or near the exit, as well as interiorly thereof which forms the build up of solidified whiskers or “stalactites” of reacted material that interferes with the material deposition on the body.
FIGS. 1A and 1B show a conventional sheet-die nozzle 10 used on a typical and conventional rotational casting machine discussed above. The sheet-die nozzle 10 includes a mixing-head attaching section 12 for securing the nozzle to a mixing head in which is contained the liquid elastomer, such as polyurethane, to be dispensed. The interior of the nozzle 10 contains a circular-cross-sectioned passageway 14 through which the liquid elastomer flows from the mixing head to the exit of the nozzle. As can be seen in FIG. 1A, the interior passageway consists of a first main line 16 which ends in an upper frustoconical-shaped entrance that immediately fluidly communicates with the exit or outlet of the mixing head. The main line 16 branches off into two branch-lines 18, 20, each of which terminates into a sheet-die slit opening 22, best seen in FIG. 1B, which slit-opening 22 extends substantially the full width of the nozzle-housing 10′. The exit of the sheet-die nozzle is a relatively elongated and narrow slit or opening, so that a wide swath of the liquid elastomer may be applied to the body to be coated, and to ensure that the drying time of the liquid is sufficiently short enough so as to prevent dripping of the applied liquid off of the element to which it has been applied. If the exiting stream of liquid material were too thick, or tall, the interior portion of the reacting liquid while still in a fluid state would not have built enough viscosity to support the column height of the stream and would run or drip off the body to which it was applied. If the reactivity were adjusted to build sufficient viscosity quickly enough to support the stream column height, the stream would not be liquid enough to flow onto the precedingly-applied material and an uneven coating would result. In a typical sheet-die nozzle 10, called a ribbon-flow nozzle manufactured by Bayer Manufacturing Co., the radius of the main passageway 16 is approximately 0.079 in., while the radius of each of the branch lines 18, 20 is approximately 0.059 in., while the slit-opening 22 has a height of approximately 0.020 in. It may, therefore, be seen that liquid material flow through the interior passageway 14 of the prior-art sheet-die nozzle 10 has considerable turbulent and boundary-layer flow characteristics, causing increased dwell-time of a hypothetical section of the flowing liquid material, which, in turn, causes increased clogging of the passageway 14 and slit-opening 22, since the greater the time any section of liquid material is present in the passageway 14, the greater the likelihood it will start to cure. This has, in fact, been one of the serious problems of the prior-art nozzle for rotational casting machines; that is, in a relatively short period of time, the nozzle becomes clogged and unusable, requiring the disassembly and cleaning thereof, which also causes considerable down-time to the rotational casting machine. Moreover, since the slit-opening 22 is fed by two branches feeding into the ends of the slit-opening, the liquid-material application onto to the body to be coated is ofttimes inconsistent and uneven, and is also limiting in the range that the distance the nozzle may be relative to the body to be coated.
FIGS. 2A and 2B show another prior-art type of nozzle 30 used in rotational casting machines. The nozzle 30 differs from the nozzle 10 of FIGS. 1A and 2B in that, in addition to the first main line 32, and two branch passageways 34, there are provided four sub-branches 36 with two extending from each branch 34, and eight capillaries 38, two from each sub-branch 36. Each capillary 38 ends in a circular outlet opening 38′ that together constitute the dispensing outlet for the nozzle 30. Thus, rather than an elongated slit-opening as in the nozzle 10 of FIGS. 1A and 1B, a series of equally-spaced openings, such as eight, are provided, through which the flowing liquid material is dispensed, as can be seen in FIG. 2B. In a typical, prior-art nozzle 30 manufactured by Uniroyal Chemical Division of Crompton Corp., the diameter of the circular-cross-sectioned main line 32 and two branches 34 is approximately 0.078 in. The diameter of each sub-branch 36 is approximately 0.063 in, while the diameter of each capillary 38 is approximately 0.047 in. Each capillary terminates into an exit hole of approximately 0.031 inch in diameter. The nozzle 30, by using equally-spaced apart dispensing holes 38′, has helped to overcome the drawback of uneven and inconsistent dispensing flow and application of the slit-opening 22 of the prior-art nozzle 10 of FIGS. 1A and 1B. However, the prior-art nozzle 30 has not addressed nor overcome the problem of consistent and frequent clogging of interior passageways described above with regard to the nozzle 10 of FIGS. 1A and 1B. In fact, owing to the narrowing of the outlet opening or holes 38′ of the nozzle 30, in some circumstances the problem with clogging and flow-impairment has been aggravated by the prior-art nozzle 30 of FIGS. 2A and 2B.
In conjunction with the need for a relatively thin exit stream of liquid material from the nozzle to ensure adequate support for the mass of the applied liquid material to the body to be coated, the rotational speed of the body being coated, and the relative translational speed between the nozzle and rotating body, must be coordinated with the speed of the liquid material exiting from the nozzle. If the rotational speed of the rotating body were to be too great in comparison to the exit speed of the liquid material from the nozzle-exit, then the applied coat may be thinner than required, and require additional coating layers to be applied to the rotating body, reducing the efficiency of the process, and also would cause air to become entrapped in the applied liquid, causing air blisters to form, since there would not be enough time for the applied stream to push out the air between the applied stream and the surface of the rotating body. On the other hand, if the rotational speed were to slow, then productivity and efficiency of the process would be adversely affected, would also increase the likelihood of premature curing, causing the eventual clogging of the nozzle, and uneven application of the coating to the rotating body. Similarly, if the relative translational motion between the exit-nozzle and the rotating body were too great, then air blisters would form, and, in addition, an applied coating of liquid material thinner than is required and optimal would be formed. Similarly, if the relative translational motion between the exit-nozzle and the rotating body were too slow, the efficiency and productivity of the process would be adversely affected, and would also cause an applied coating that would be too thick, thus causing dripping of the applied liquid from the body being coated, as well as potentially uneven thickness of the applied coat.
The need and requirement for optimal correspondence between exit speed of the liquid from the nozzle, the thickness of the exiting stream of liquid, the rotational speed of the rotating body being coated relative to this exit speed of the liquid from the nozzle, and the relative translational speed between the nozzle and the rotating body being coated has imposed significant constraints as to linear distance the exit of the nozzle of the rotating casting machine may be from the surface of the rotating body being coated. Presently-used rotational casting machines provide an outer limit of only approximately 5 mm. of the nozzle-exit from the surface of the rotating body being coated. A distance greater than 5 mm. has been found to cause excessive clogging of the nozzle, with a concomitant increase of downtime of the machine for unclogging the nozzle. This excessive clogging ensues from the fact that as the nozzle-exit distance from the surface to be coated is increased, the exit-speed of the liquid must be increased in order to compensate therefore. The increase in speed of the liquid through the nozzle increases turbulent flow in the nozzle, thus increasing the dwell-time of the liquid in the nozzle, and the increased curing thereof in the nozzle, with the ensuing clogging of the nozzle, as discussed hereinabove. Besides the increased clogging of the nozzle, air blisters form in the applied coating of liquid, for the reasons described hereinabove due to the increased exit speed of the liquid from the nozzle-exit.
Another considerable problem with the sheet-die nozzle of FIG. 1 is that the size of the rotating body that may be coated with the liquid exiting therefrom is limited. Cylindrical bodies having a diameter less than approximately five inches have not been able to effectively coated with liquid. This is because of the requirement described above for correlation between the speed of the rotational body to be coated, the exit-speed of the liquid from the nozzle-exit, and the turbulent flow of the liquid in the nozzle proper and the increased dwell-time of the liquid in the nozzle associated therewith.